In a control scheme carried out in image display apparatus using light valves such as liquid crystal panels, the brightness of the backlight or other light source is adjusted responsive to the image signal. The image display apparatus disclosed in patent document 1 below adjusts the brightness of the light source responsive to changes in the DC level of the image so as not to change the average brightness level of the displayed image due to changes in the DC level of the image that occur when the contrast is adjusted. This scheme improves the contrast of the displayed image.    Patent document 1: Japanese Patent No. 3215388